The current state-of-the-art fiber lasers and fiber amplifiers have difficulty in amplifying laser pulses to very large power levels such that the output pulse lasts for about one microsecond or longer, and outputs several millijoules of energy while also avoiding undesired non-linear effects such as stimulated Brillouin scattering (SBS). SBS is exacerbated by long pulse duration (pulses longer than about 5 nanoseconds (nsec)) and by very narrow linewidths (laser signals having a relatively broad linewidth are less likely to cause SBS degradation).
Optical gain fibers doped with rare-earth dopants (such as erbium-doped fiber amplifiers (EDFAs)) enable laser designs where the optical gain fiber is optically pumped over an extended period of time (e.g., 100's or 1000's of microseconds) in order to accumulate a relatively large amount of energy, and an optical seed pulse (a lower-power laser pulse from a laser source) is then launched into the waveguide core of the optical gain fiber to extract the accumulated energy by stimulated-emission amplification. Unfortunately, this typically results in pulse steepening, since the leading edge of the seed pulse encounters the highest gain, while later temporal portions of the pulse will undergo lower amounts of amplification.
One approach to solving the pulse-steepening problem is to shape the seed pulse by amplitude modulation (providing a low-amplitude leading edge for the seed pulse followed by a rising amplitude later in the pulse), but it is quite difficult to provide sufficient dynamic range and fine control to obtain satisfactory energy extraction, pulse shape, and avoidance of non-linear effects. Even when a fiber amplifier or fiber laser is designed to compensate for the above effects, there will be a limit on the maximum power that can be obtained from a single fiber when scaling to larger fiber sizes and/or lengths, pump powers, and the like.
Various inventions use spectral-beam combining. U.S. Pat. No. 6,192,062 to Sanchez-Rubio et al. entitled “Beam combining of diode laser array elements for high brightness and power” and U.S. Pat. No. 6,208,679 to Sanchez-Rubio et al. entitled “High-power multi-wavelength external cavity laser” describe the fundamental techniques of spectral beam combining, and both are incorporated herein by reference.
In some embodiments, the gratings used for spectral-beam combining are “blazed,” i.e., formed with V-grooves having sidewall angles that are asymmetrical with respect to a vector normal to the overall surface of the grating. U.S. Pat. No. 3,728,117 to Heidenhain et al. entitled “Optical Diffraction Grid” (incorporated herein by reference) describes a method for making blazed gratings having asymmetric grooves. U.S. Pat. No. 4,895,790 to Swanson et al. entitled “High-efficiency, multilevel, diffractive optical elements” (incorporated herein by reference) describes a method for making blazed gratings having asymmetric grooves using binary photolithography to create stepped profiles. U.S. Pat. No. 6,097,863, titled “Diffraction Grating with Reduced Polarization Sensitivity” issued Aug. 1, 2000 to Chowdhury (incorporated herein by reference) describes a reflective diffraction grating with reduced polarization sensitivity for dispersing the signals. The Chowdhury grating includes facets that are oriented for reducing efficiency variations within a transmission bandwidth and that are shaped for reducing differences between the diffraction efficiencies in two orthogonal directions of differentiation. U.S. Pat. No. 4,313,648 entitled “Patterned Multi-Layer Structure and Manufacturing Method” issued Feb. 2, 1982 to Yano et al. (incorporated herein by reference) describes a manufacturing method for a patterned (striped) multi-layer article.
U.S. Pat. No. 6,822,796 to Takada et al. titled “DIFFRACTIVE OPTICAL ELEMENT” (incorporated herein by reference) describes a method for making blazed gratings having asymmetric grooves with dielectric coatings. U.S. Pat. No. 6,958,859 to Hoose et al. entitled “Grating device with high diffraction efficiency” (incorporated herein by reference) describes a method for making blazed gratings having dielectric coatings.
U.S. Pat. No. 5,907,436 titled “MULTILAYER DIELECTRIC DIFFRACTION GRATINGS” issued May 25, 1999 to Perry et al., and is incorporated herein by reference. This patent describes the design and fabrication of dielectric grating structures with high diffraction efficiency. The gratings have a multilayer structure of alternating index dielectric materials, with a grating structure on top of the multilayer, and obtain a diffraction grating of adjustable efficiency, and variable optical bandwidth.
U.S. Pat. No. 6,212,310 titled “HIGH POWER FIBER GAIN MEDIA SYSTEM ACHIEVED THROUGH POWER SCALING VIA MULTIPLEXING” issued 3 April 2001 to Waarts et al., and is incorporated herein by reference. This patent describes certain methods of power scaling by multiplexing multiple fiber gain sources with different wavelengths, pulsing or polarization modes of operation is achieved through multiplex combining of the multiple fiber gain sources to provide high power outputs, such as ranging from tens of watts to hundreds of watts, provided on a single mode or multimode fiber.
U.S. Pat. No. 7,532,656 issued to Yang, et al. on May 12, 2009 titled “ALL-SILICON RAMAN AMPLIFIERS AND LASERS BASED ON MICRO RING RESONATORS” and is incorporated herein by reference. This patent describes devices for generating a laser beam. The devices include a silicon optical micro-ring having at least one silicon optical waveguide disposed at a distance from the micro-ring. The radius and the cross-sectional dimension of the micro-ring, the cross-sectional dimension of the waveguide, and the distance between the micro-ring and the waveguide are determined such that one or more pairs of whispering-gallery-mode resonant frequencies of the micro-ring are separated by an optical-phonon frequency of silicon.
U.S. Pat. No. 6,330,388 issued to Bendett, et al. on Dec. 11, 2001 titled “METHOD AND APPARATUS FOR WAVEGUIDE OPTICS AND DEVICES,” and U.S. Pat. No. 6,636,678 issued to Bendett, et al. on Oct. 21, 2003, also titled “METHOD AND APPARATUS FOR WAVEGUIDE OPTICS AND DEVICES,” and both are incorporated herein by reference. These patents describe optical structures and methods for producing tunable-waveguide lasers. In one embodiment, a waveguide is defined within a glass substrate doped with a rare-earth element or elements by ion diffusion. Feedback elements such as minors or reflection gratings in the waveguide further define a laser-resonator cavity so that laser light is output from the waveguide when pumped optically or otherwise. Means are disclosed for varying the wavelengths reflected by the reflection gratings and varying the effective length of the resonator cavity to thereby tune the laser to a selected wavelength. These patents also describe apparatus and method for integrating rare-earth-doped lasers and optics on glass substrates.
U.S. Pat. No. 6,970,494 issued to Bendett, et al. on Nov. 29, 2005 titled “Rare-earth doped phosphate-glass lasers and associated methods” and is incorporated herein by reference. This patent describes integrating lasers and optics on glass substrates. An optical (e.g., laser) component formed from a glass substrate doped with an optically active lanthanides species with a plurality of waveguides defined by channels within the substrate. The laser component optionally includes a monolithic array of individual waveguides in which the waveguides form laser resonator cavities with differing resonance characteristics.
U.S. Pat. No. 6,813,405 issued to Bendett, et al. on Nov. 2, 2004 titled “Compact apparatus and method for integrated photonic devices having folded directional couplers” and is incorporated herein by reference. This patent describes an integrated photonic apparatus that includes a glass substrate having a major surface, a first waveguide segment and a second waveguide segment, and a folded evanescent coupler connecting the first waveguide segment to the second. The folded evanescent coupler is formed by a first length of the first waveguide segment and an equivalent length portion of the second waveguide running parallel and adjacent to the first waveguide segment. The first length is substantially equal to one half of an evanescent-coupler length needed to transfer a first wavelength in a non-folded evanescent coupler. A reflector (e.g., dielectric mirror that is highly reflective to light of the first wavelength and also highly transmissive to light of a second wavelength) is located at an end of the folded evanescent coupler.
U.S. Pat. No. 6,493,476 issued to Bendett on Dec. 10, 2002 titled “APPARATUS AND METHOD FOR INTEGRATED PHOTONIC DEVICES HAVING GAIN AND WAVELENGTH-SELECTIVITY” and is incorporated herein by reference. This patent describes an integrated photonic apparatus that includes a glass substrate having a major surface, wherein the glass substrate includes a plurality of regions, each region having a different index of refraction, including a first region having a first index of refraction and a second region having a second index of refraction lower than the first index of refraction, and a first waveguide formed along the major surface of the substrate, wherein the first waveguide has a higher index of refraction than an intrinsic index of refraction of adjacent portions of the substrate, and wherein the first waveguide passes through the first region and through the second region of the glass substrate.
U.S. Pat. No. 7,403,677, to Zhao, et al., which issued Jul. 22, 2008 titled “Fiberoptic reconfigurable devices with beam shaping for low-voltage operation,” is incorporated herein by reference. U.S. Pat. No. 7,403,677 describes an apparatus and method to operate on a light beam by using a lens that collimates the light beam to a collimated beam with at least one cross-sectional dimension less than a critical dimension of 400 microns or less over a working range WR. The apparatus has a bulk electro-optic material of small thickness, e.g., less than 300 microns positioned within a working range and the collimated beam traverses it along its path. The apparatus has a voltage source for applying a low operating or drive voltage, e.g. less than 400 V, to the bulk electro-optic material for performing an operation on the collimated beam. The lens for collimating the light beam is a free-space collimator such as a graded index (GRIN) lens or preferably a C-lens. U.S. Pat. No. 4,778,237 to Sorin, et al. issued Oct. 18, 1988 titled “Single-mode fiber optic saturable absorber” is incorporated herein by reference. U.S. Pat. No. 4,778,237 describes fiber optic saturable absorber for processing optical signals comprises an optical fiber from which a portion of the cladding is removed to form a facing surface. A light-absorbing substance having non-linear light-absorbing characteristics is applied to the facing surface such that a portion of the optical signal energy is transferred from the fiber to the substance where it is absorbed. The device selectively attenuates the optical signal and noise, and can be used to reduce pulse waveform distortion caused by pulse broadening and by amplification of system noise.
U.S. Pat. No. 6,396,975 to Wood et al. issued May 28, 2002 titled “MEMS optical cross-connect switch” and is incorporated herein by reference. U.S. Pat. No. 6,396,975 describes a MEMS (microelectromechanical) structure capable of switching optical signals from an input fiber to one of two or more output fibers. In one embodiment, the MEMS optical cross-connect switch comprises a first microelectronic substrate having a pop-up mirror disposed on the surface of the substrate and a rotational magnetic field source, such as a variably controlled magnetic field source. The rotational magnetic field source allows for reliable actuation of the pop-up minor from a non-reflective state to a reflective state. Additionally the invention is embodied in a MEMS optical cross-connect switch having a first microelectronic substrate having a pop-up minor disposed on the surface of the substrate and a positioning structure disposed in a fixed positional relationship relative to the first substrate. The positioning structure may comprise a positioning structure extending from a second microelectronic substrate that is in a fixed positional relationship relative to the first microelectronic substrate. The positioning structure serves to restrict further movement of the pop-up minor when the pop-up minor has been actuated into a reflective state.
U.S. Pat. No. 4,778,237 to Sorin, et al. Oct. 18, 1988 “Single-mode fiber optic saturable absorber”, is incorporated herein by reference. U.S. Pat. No. 4,778,237 describes fiber optic saturable absorber for processing optical signals comprises an optical fiber from which a portion of the cladding is removed to form a facing surface. A light-absorbing substance having non-linear light-absorbing characteristics is applied to the facing surface such that a portion of the optical signal energy is transferred from the fiber to the substance where it is absorbed. The device selectively attenuates the optical signal and noise, and can be used to reduce pulse waveform distortion caused by pulse broadening and by amplification of system noise.
U.S. Pat. No. 7,203,209 to Young, et al. Apr. 10, 2007 titled “System and method for a passively Q-switched, resonantly pumped, erbium-doped crystalline laser”, is incorporated herein by reference. U.S. Pat. No. 7,203,209 describes a laser that includes a resonant cavity formed between a first mirror and a second mirror. An unsensitized erbium-doped crystal gain medium for producing laser gain is disposed within the resonant cavity. A saturable absorber is disposed within the resonant cavity. A pump source is positioned to energize the gain medium. The saturable absorber, the laser gain, the resonator length, and the second minor being selected so that output pulses having a duration of less than 75 nanoseconds are generated by the laser.
The present invention can be used with or combined with any of the prior-art patents described herein to obtain novel and non-obvious combinations, including spectral-beam-combined laser beams from fiber lasers for directed energy (DE) weapons, for example as being proposed for the U.S. robust electric-powered laser initiative (RELI). In some embodiments, the present invention produces a high-power laser that pumps Raman-fiber amplifiers or lasers for DE at eye-safer wavelengths.
There is a need for improved laser systems, particularly Q-switched fiber lasers and/or fiber optical amplifiers for use in MOPA designs. While other fiber-laser alternatives are available, the present invention provides improved performance (higher output power) and/or lower cost.